The present invention relates generally to location devices and more specifically to methods and apparatus used to locate passive integrated transponder tags (hereinafter “PIT tags”).
Location devices, such as metal detectors and radio frequency transponder locators for tracking objects or specimens such as animals are known in the art. However, these devices have drawbacks that make them unsuitable for some tasks. For example, radio frequency transponder tags are intended for location and tracking of objects or animals at comparatively long range, and not for close-range location of small objects with great precision. Metal detectors react to any substantial amount of metal found within an object, and therefore do not differentiate between PIT tags and metal objects, or multiple devices embedded in a single object. They are also incapable of the precision required for certain applications. Thus, when it is necessary to identify a location of a single embedded device to within a few millimeters, and/or it is desirable to differentiate between multiple implanted devices, PIT tags are preferred.
PIT tags have been used for many years to identify specimens, including livestock, domestic pets, birds, fish, and other marine animals for various management and/or research purposes. Each PIT tag generally includes a small ferrite-cored coil attached to a microchip. The microchip has a capacitor that causes the coil to resonate at a predetermined frequency when energized and circuitry to generate and transmit a coded identification number or message in response to a received interrogation signal which energizes the coil. PIT tags do not contain an internal energy source. Instead, energy needed to transmit the coded identification number is obtained through electromagnetic coupling, which causes a transfer of energy from a powered device to the PIT tag. Typically, the PIT tag is enclosed in a glass covering or envelope about 2 mm in diameter and about 11 mm in length, although other packaging is possible. PIT tags are usually injected up to a few centimeters below the outer surface of an object or the specimen's skin using a hypodermic syringe, but other methods of attachment, for example ear tags, are also known.
Protocols for a PIT tag interrogation and messaging system include those defined by International Standards Organization (ISO) standards 11784 and 11785, and other protocols that have been introduced by various manufacturers. PIT tags may be either half-duplex (HDX) or full-duplex (FDX). An HDX PIT tag receives a pulsed interrogation signal from a “PIT tag reader” and then responds with a coded identification number. An FDX PIT tag continuously transmits a coded identification number while receiving an interrogation signal, which may be either pulsed or continuous. PIT tags are typically read at close range, generally well under 1 meter, and often less than a few centimeters.
In many applications, the embedded PIT tags remain in position for the life of the specimen or object and are treated as disposable items. One known limitation of PIT tag readers, which are devices that are capable of receiving the coded identification number transmitted by the PIT tag and displaying the number, is that the readers cannot accurately determine the position of the PIT tag after it has been embedded within an object or specimen. However, in certain situations it is beneficial to be able to accurately determine the location of the PIT tag in terms of the depth of the PIT tag relative to the object's surface and/or the location on the surface of the object below which the PIT tag is embedded. For example, knowing the exact position of a PIT tag is helpful to reduce damage to an object or specimen if it is necessary to remove the PIT tag from the object or specimen. Knowledge of a PIT tag's position is also beneficial if the location of the PIT tag is used as a marker for some other device having the PIT tag attached or adjacent thereto and which is also disposed in the object or specimen. Thus, there is a need for a PIT tag locating device that is able to determine the position and depth of a PIT tag that has been embedded in an object or specimen in addition to receiving a coded identification number transmitted by the PIT tag.